At least one copy of every American film is required to be deposited and catalogued at the Library of Congress for copyright reasons. Although U.S. copyright law required copies to be deposited at the time of copyright registration, the Librarian of Congress was not required to retain those copies. "Under the provisions of the act of March 4, 1909, authority is granted for the return to the claimant of copyright of such copyright deposits as are not required by the Library."[1] Of American silent films far more have been lost than have survived, and of American sound films made from 1927 to 1950, perhaps half have been lost.[2] The phrase "lost film" can also be used in a literal sense for instances where footage of deleted scenes, unedited and alternative versions of feature films are known to have been created but can no longer be accounted for. Sometimes a copy of a lost film is rediscovered. A film that has not been recovered in its entirety is called a partially lost film. For example, the 1922 film Sherlock Holmes was eventually discovered but some of the footage is still missing.

Most film studios routinely had a "still" photographer with a large-format camera working on the set during production, taking pictures for potential later publicity use. The high-quality photographic paper prints that resulted – some produced in quantity for display use by theaters, others in smaller numbers for distribution to newspapers and magazines – have preserved imagery from many otherwise lost films. In some cases, such as London After Midnight, the surviving coverage is so extensive that an entire lost film can be reconstructed scene by scene in the form of still photographs. Stills have been used to stand in for missing footage when making new preservation prints of partially lost films.

The largest cause of silent film loss was intentional destruction, as silent films were perceived as having little or no commercial value after the end of the silent era by 1930. Film preservationist Robert A. Harris has said, "Most of the early films did not survive because of wholesale junking by the studios. There was no thought of ever saving these films. They simply needed vault space and the materials were expensive to house."[6]

Many other early motion pictures are lost because the nitrate film used for nearly all 35 mmnegatives and prints made before 1952 is highly flammable. When in very badly deteriorated condition and improperly stored (e.g., in a sun-baked shed), nitrate film can even spontaneously combust. Fires have destroyed entire archives of films. For example, a storage vault fire in 1937 destroyed all the original negatives of Fox Pictures' pre-1935 films.[7] A 1967 MGM Vault fire resulted in the loss of hundreds more silent films and early talkies. Nitrate film is chemically unstable and over time can decay into a sticky mass or a powder akin to gunpowder. This process can be very unpredictable: some nitrate film from the 1890s is still in good condition today, while some much later nitrate had to be scrapped as unsalvageable when it was barely twenty years old. Much depends on the environment in which it is stored. Ideal conditions of low temperature, low humidity and adequate ventilation can preserve nitrate film for centuries, but in practice the storage conditions were usually far from ideal. When a film on nitrate base is said to have been "preserved", this almost always means simply that it has been copied onto safety film or, more recently, digitized; both methods result in some loss of quality.

Eastman Kodak introduced a nonflammable 35 mm film stock in spring 1909. However, the plasticizers used to make the film flexible evaporated too quickly, making the film dry and brittle, causing splices to part and perforations to tear. By 1911 the major American film studios were back to using nitrate stock.[8] "Safety film" was relegated to sub-35 mm formats such as 16 mm and 8 mm until improvements were made in the late 1940s.

Some pre-1931 sound films made by Warner Bros. and First National have been lost because they used a sound-on-disc system with a separate soundtrack on special phonograph records. If some of a film's soundtrack discs could not be found when 16 mmsound-on-filmreduction prints of early "talkies" were being made for television use in the 1950s, that film's chances of survival plummeted: many sound-on-disc films have survived only by way of those 16 mm prints.

Before the eras of television and later home video, films were viewed as having little future value when their theatrical runs ended. Thus, again, many were deliberately destroyed to save the space and cost of storage; many were recycled for their silver content. Many Technicolor two-color negatives from the 1920s and 1930s were thrown out when the studios refused to reclaim their films, still being held by Technicolor in its vaults. Some prints were sold either intact or broken into short clips to individuals who bought early novelty home projection machines and wanted scenes from their favorite movies to play for guests or family members.

As a consequence of this widespread lack of care, the work of many early filmmakers and performers has made its way to the present in fragmentary form. A high-profile example is the case of Theda Bara. One of the best-known actresses of the early silent era, she made 40 films, but only three and a half are now known to exist. Clara Bow was equally celebrated in her heyday, but twenty of her 57 films are completely lost and another five are incomplete.[9] Once-popular stage actresses such as Pauline Frederick and Elsie Ferguson who made the jump to silent films are now largely forgotten with a minimal archive to represent their careers; fewer than ten movies exist from Frederick's 1915-1928 work, and Ferguson has just two surviving films, one from 1919 and one from 1930. This is preferable to the fate of the stage actress and Bara rival Valeska Suratt, whose entire film career has been lost. Western hero William Farnum, a Fox player like Bara and Suratt, was one of the four big Western actors rivaling William S. Hart, Tom Mix and Harry Carey. Farnum has about three of his Fox films extant. Other male performers like Francis X. Bushman and William Desmond had numerous film credits, but films made in their heyday are missing due to junking, neglect or studios being defunct. Nevertheless, unlike Suratt and Bara, these men continued working into the sound era and even into television, thus their later performances can be observed and appreciated.

There are occasional exceptions. Almost all of Charlie Chaplin's films from his entire career have survived as well as extensive amounts of unused footage dating back to 1916. The exceptions are A Woman of the Sea (which he destroyed himself as a tax writeoff) and one of his early Keystone films, Her Friend the Bandit (see Unknown Chaplin). The filmography of D.W. Griffith is nearly complete, as many of his early Biograph films were deposited by the company in paper print form at the Library of Congress. Much of Griffith's feature film work of the 1910s and 1920s found their way to the film collection at the Museum of Modern Art in the 1930s and were preserved under the auspices of curator Iris Barry. Mary Pickford's filmography is very much complete: her early years were spent with Griffith, and later she gained control of her own productions in the late 1910s and early 1920s. She also backtracked to as many of her Zukor-controlled early Famous Players films that were salvageable. Stars like Chaplin and Douglas Fairbanks enjoyed stupendous popularity, and their films were reissued over and over throughout the silent era, meaning prints of their films were likely to surface decades later. Pickford, Chaplin, Harold Lloyd and Cecil B. DeMille were early champions of film preservation. Lloyd lost a good deal of his silent work in a vault fire in the early 1940s.

One remarkable case was the 1919 German film Different from the Others (Anders als die Andern), starring Conrad Veidt. A striking plea for tolerance for homosexuality, produced in collaboration with Dr. Magnus Hirschfeld, it was targeted for destruction by the Nazis, with many prints of the film burned as decadent. However, a 50-minute fragment survived the censorship attempt.

An improved 35 mm safety film was introduced in 1949. Since safety film is much more stable than nitrate film, there are comparatively few lost films after about 1950. However, color fading of certain color stocks and vinegar syndrome threaten the preservation of films made since about this time.

Most mainstream movies from the 1950s onwards survive today, but several early pornographic films and some B-Movies are lost. In most cases these obscure films go unnoticed and unknown, but some films by noted cult directors have been lost as well:

Several films by Kenneth Anger from across his career have been lost for a variety of reasons.

Ed Wood's 1972 film, The Undergraduate, has been lost along with his 1970 film Take It Out In Trade, which exists only in fragments without sound. Wood's 1971 film Necromania was believed lost for years until an edited version resurfaced at a yard sale in 1992, followed by a complete unedited print in 2001.[10] A complete print of the previously lost Wood pornographic film The Young Marrieds was discovered in 2004.

The first three films of noted Finnish melodrama actor and director Teuvo Tulio were lost along with several other films that were of interest at least for historians of Finnish cinema, when the film depository of the company Adams Filmi burnt down in Helsinki in 1959.

Some films produced in 1926–1931 in sound-on-disc systems such as Vitaphone, where the sound discs are separate from the film element, are now considered lost because the sound discs were damaged or destroyed, while the picture element was not. Conversely, some Vitaphone-produced movies survive only as sound, with the film element missing (such as 1930's The Man from Blankley's, starring John Barrymore).

Many stereophonic soundtracks from the early to mid-1950s that were either played in interlock on a 35 mm fullcoat magnetic reel or single-strip magnetic film (such as Fox's four-track magnetic, which became the standard of mag stereophonic sound) are now lost. Films such as House of Wax, The Caddy, The War of the Worlds, The 5,000 Fingers of Dr. T, and From Here to Eternity that were originally available with 3-track, magnetic sound are now available only with a monophonic optical soundtrack. The chemistry behind adhering magnetic particles to the tri-acetate film base eventually caused the autocatalytic breakdown of the film (vinegar syndrome). As long as studios had a monaural optical negative that could be printed, studio executives felt no need to preserve the stereophonic versions of the soundtracks.

Beyond the Rocks (1922) with Gloria Swanson and Rudolph Valentino was considered a lost film for several decades. Swanson lamented the loss of this and other films in her 1980 memoirs, but optimistically concluded, "I do not believe these films are gone forever". In 2000, a print was found in the Netherlands and restored by the Nederlands Filmmuseum and the Haghefilm Conservation. It turned up among about 2000 rusty film canisters donated by an eccentric Dutch collector, Joop van Liempd, of Haarlem. It was given its first modern screening in 2005, and has since been aired on Turner Classic Movies.

In the early 2000s, the 1927 German film Metropolis—which had been distributed in many different edits over the years—was restored to as close to the original version as possible by reinstating edited footage and using computer technology to repair damaged footage. At that point, however, approximately a quarter of the original film footage was considered lost, according to Kino Video's DVD release of the restored film. On July 1, 2008 Berlin film experts announced that a copy of the film had been discovered in the archives of the film museum Museo del Cine in Buenos Aires, Argentina, which contained all but one of the scenes still missing from the 2002 restoration.[12][13] The film now has been restored very close to its premiere version.

Television material existing on film has sometimes been recovered. The 1951 pilot of I Love Lucy was long believed lost, but in 1990 the widow of one of the actors, Pepito Pérez (who played Pepito the Clown), found a copy. It has since been shown on television. Sometimes a film believed lost in its original state has been restored, either through the process of colorization, or other restoration methods. The Cage, the original 1964 pilot film for Star Trek, survived only in a black-and-white print until 1987, when a film archivist found an unmarked (mute) 35mm reel in a Hollywood film laboratory with the negative trims of the unused scenes.[15]

Similarly, a number of videotaped television programmes, previously thought lost (see wiping) have been recovered as overseas Kinescope film prints from private collectors and various other sources over the years.

Several films have been made with lost film fragments incorporated into the work. Decasia (2002) used nothing but decaying film footage as an abstract tone poem of light and darkness, much like Peter Delpeut's more historical Lyrisch Nitraat (Lyrical Nitrate, 1990) which contained only footage from canisters found stored in an Amsterdam cinema. In 1993, Delpeut released The Forbidden Quest, combining early film footage and archival photographs with new material to tell the fictional story of an ill-fated Antarctic expedition.

The James Cagney film Winner Take All (1932) used scenes from the early talkie Queen of the Night Clubs (1929) starring Texas Guinan. While Queen of the Night Clubs was not a lost film in 1932, no prints of the film have survived through the decades since then. But the Cagney movie still is extant along with the selected footage taken from Queen of the Night Clubs.

Actress turned gossip columnist Hedda Hopper made her screen debut in a Fox Film called The Battle of Hearts (1916). The star of the film was William Farnum, then at the beginning of his long Fox contract. 26 years later in 1942 Hopper produced her short series Hedda Hopper's Hollywood #2. In the short, Hopper, Farnum, her son William Hopper, and William's wife Jane Gilbert view portions of Battle of Hearts. These brief portions of that movie survive within the Hopper documentary. More than likely Hopper had an entire print of the movie in 1942. However, like many early Fox films, Battle of Hearts is now lost or missing.

One of Charlie Chaplin's best-known works, the 1925 silent film The Gold Rush, was remade in 1942 to include a musical track and narration by Chaplin himself. The remake would end up having the unintentional result of preserving the film, as the original 1925 film (though generally not considered a lost film) shows noticeable degradation of image and missing frames, damage not in evidence in the 1942 version.

Polish film O czym się nie mówi from 1939 contains three short fragments of Arabella from 1917, one of early Pola Negri's film, which later became lost.